The present invention generally relates to a metallization for aluminum nitride ceramics having properties of a low thermal expansion and a high thermal conductivity. More particularly, the present invention relates to a metallization layer structure formed on aluminum nitride ceramics and a method of producing the metallization layer structure on the aluminum nitride ceramics.
Conventionally, alumina (Al.sub.2 O.sub.3) ceramics, or a material of an oxide system such as glass containing silicon dioxide (SiO.sub.2) as a major component, are used as structural elements of various electronic devices such as packages of semiconductor integrated circuit devices. Normally, there is a need for fixedly fastening an structural part such as a pin for an external connection to alumina ceramics or glass of the oxide system. Conventionally, a metallization is employed such that a metallization layer is formed on alumina ceramics or glass of the oxide system. The metallization layer functions as an adhesive layer, so that structural parts can be fixedly fastened to alumina ceramics or glass of the oxide system. Normally, chromium (Cr), titanium (Ti) or aluminum is used as a material for the metallization with respect to alumina ceramics or glass of the oxide system. Structural parts are mounted on the metallization layer formed alumina ceramics or glass of the oxide system, and are then soldered or brazed. The above metallization has been disclosed in the Japanese Laid-Open Patent Application Nos. 79910/1978, 207691/1984 and 57997/1985, for example.
Recently, there has been considerable activity in use of aluminum nitride (AlN) ceramics as ceramics having properties of a small thermal expansion coefficient and a high thermal conductivity, compared with Al.sub.2 O.sub.3 ceramics. The thermal conductivity of AlN ceramics is also smaller than that of glass of the SiO.sub.2 system. AlN ceramics have a thermal expansion coefficient of 4.times.10.sup.-6 /.degree.C., which is about half the thermal expansion coefficient of Al.sub.2 O.sub.3 ceramics, or 7.times.10.sup.-6. The thermal conductivity of AlN ceramics is three to eight times that of Al.sub.2 O.sub.3 ceramics, and is also more than one thousand times as large as that of SiO.sub.2 glass.
However, the Cr, Ti or Al metallization layer does not have a sufficient adhesive strength with respect to AlN ceramics of the non-oxide system. Therefore, the conventional metallization layer may be broken away from AlN ceramics. This lead to degradation in a hermetic seal of a package, for example.